1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method, information processing apparatus, and information processing method, that use, for example, super-resolution techniques.
2. Description of the Related Art
Digital copiers, which read an image of a document using a scanner and make copies using the obtained image data, and image processing systems, which analyze image data and convert that image data into document data, are available as of late. Due to variance in the properties, optical processing, and so on of the scanning elements, scanners that use image sensors such as CCDs, CISs, and so on sometimes experience differences in scanning resolutions, or in other words, differences in Modulation Transfer Functions (MTFs) at the individual scanner unit level.
When the MTF is low, image objects that should have high resolution, such as small characters and the like, experience a drop in image quality, resulting in the characters scanned by the scanner being unreadable, distorted, and so on. For this reason, there are cases where optical character recognition (OCR) carried out on the scanned data cannot successfully extract the characters.
The makers of image processing apparatuses have conventionally set standards of quality with respect to MTFs in scanners and shipped, as products, only scanner devices that meet those standards.
Meanwhile, attempts are being made to use image processing to correct images for which the resolution has dropped and increase the MTF. This correction is called “MTF correction”, and many different methods thereof have been proposed.
For example, in Japanese Patent Laid-Open No. 2000-307870 (Patent Document 1), MTF correction is performed by using an MTF correction chart to calculate MTF correction parameters and obtain an MTF filter. In Japanese Patent Laid-Open No. 2005-45433 (Patent Document 2), meanwhile, adjusts the MTFs by adjusting the focus of the image-forming optical system, adjusting the filter coefficients of spatial filters, and so on in order to suppress variations in MTFs between the R, G, and B channels. Finally, Japanese Patent Laid-Open No. 2007-150517 (Patent Document 3) attempts to retain a ground design component by correcting, in accordance with the reading of a ground design pattern, the MTF of the image data based on the degree to which a dotted pattern can be detected in the read image data.
Meanwhile, a technique known as “super-resolution”, which improves resolution by using multiple low-resolution images, exists as a technique for correcting resolution (see, for example, WO 2004/068862 (Patent Document 4)). Super-resolution processing is a technique that generates a high-resolution image by extracting subpixels using multiple pieces of image data, each with different phases, obtained from a signal image. Improving the accuracy of this subpixel extraction makes it possible to obtain an image of higher resolution. The accuracy of the extracted subpixels increases proportionally with the number of pieces of low-resolution image data with different phases that are employed.
MTF correction using conventional MTF filters, as described above, does not actually increase the resolution of the image data itself, and is thus limited in how much it can improve resolution. There is also a problem with systems that use multiple scanners in that the results of processing read image data may differ due to variances in the MTFs at the scanner level. For example, there are cases where the results obtained by running an OCR process on a certain document image using a certain multifunction peripheral device (MFP) do not match the results obtained by running an OCR process on the same document image using a different MFP of the same type. This is because the MTFs differ due to individual differences in the scanners.
Furthermore, there is a problem in that the MTF of a scanner will drop due to deterioration in the light source, image sensor elements, and so on occurring over time as the scanner unit is used. The results of scanning processes performed on a certain document image will therefore also deteriorate over time in accordance with the stated deterioration in the MTF. There is therefore a problem in that, for example, characters that were once recognizable when a certain MFP was new become unreadable as time passes, leading to an increase in recognition errors and so on.